1. Field of the Invention
The present invention relates to a communication apparatus that can directly communicate with another communication apparatus via a wireless communication, and a control method thereof.
2. Description of the Related Art
In recent years, streaming distribution systems such as video conferencing systems, Video on Demand (VoD) systems and the like using Internet Protocol (IP) networks are prevalent. In such streaming distribution systems, upon playing back, audio, video, and image data in real time, Transport Control Protocol (TCP), which executes re-send control upon discarding packets or occurrence of data errors, normally can hardly maintain real-time delivery.
Consequently, a system such as a video conferencing system, VoD system, and the like that assumes a plurality of communication nodes normally uses User Datagram Protocol (UDP) in place of TCP. However, since a UDP header does not have any field for storing sequence numbers that indicate the order of packets, unlike a TCP header, it is difficult to attain control for accurately re-arranging the changed order, if the order of packets has changed on the network.
In order to solve the problem, Real-time Transport Protocol (RTP) has been proposed as a transport protocol used to exchange audio and video data in real time on the IP network. Also, RTP Control Protocol (RTCP) has been proposed as a control program used to exchange audio and video data in real time.
Normally, the RTP is used on the UDP, and has many advantages: it allows order correction of packets on the data receiving side, can absorb delay fluctuations on the network, and so forth. For this reason, the RTP is popularly known as a technique for playing back audio, video, and image data in real time in a video conferencing system, VoD system, and the like.
As described above, RTP/RTCP is generally used as a technique for assuring quality of service (QoS) on layers higher than the transport layer.
RTP, as a technique for assuring QoS, can append RTP time stamps, which are accurately synchronized with time stamps (PCR) of TS packets used upon transferring video data of, e.g., MPEG-TS or the like; for example, see patent reference 1: Japanese Patent Laid-Open No. 2001-320413.
As approaches to QoS on the Internet, responding to the advent of stream-oriented applications, studies were first made in 1989, and the standardization of the following techniques has been underway since that time:                Integrated Service (IntServ): the Internet Engineering Task Force (IETF) working group, and a technique for making QoS control for respective application flows; and        Resource reSerVation Protocol (RSVP): a technique for dynamically assuring resources on a communication channel as needed.        
As QoS techniques different from IntServ, the following techniques are available:                Differentiated Service (DiffServ): a technique for classifying and grouping application packets, defining priority levels for the groups, and making data transfer in accordance with the priority levels; and        a technique using a VLAN tag header based on a priority tag that is compliant with IEEE 802.1p. The tag header includes a 3-bit user priority field, and the priority level to be handled in the network is determined by the field value for each packet.        
In order to transmit video stream data wirelessly, IEEE 802.11e has considered introducing a concept of “QoS” in a wireless transfer, and guaranteeing the priority order and communication bandwidth, depending on the data content and intended purposes thereof. The IEEE 802.11e protocol specifies the control method not only for communication between terminals via a wireless access point, but also for a direct communications link between the terminals for the purpose of effective use of bandwidth. Such direct communications are called Direct Link Setup (DLS).
However, an adapter device used to accommodate a device that comprises an input/output function of communication data, such as audio data and video data that require real-time delivery in the IP network environment, poses the following problem in terms of services:
Since the support status of a QoS protocol, e.g., RTP/RTCP, for data exchanged between devices cannot be recognized, an unwanted QoS-related protocol is unnecessarily appended to data. Upon requiring a QoS-related protocol, for example, when a device does not have any support function of the QoS protocol, it does not append any QoS-related protocol.